Heterogeneously catalyzed partial gas phase oxidation of propene to acrolein

ABSTRACT

In a process for heterogeneously catalyzed partial gas phase oxidation of propene to acrolein, the starting reaction gas mixture is oxidized over a fixed catalyst bed which consists of at least two fixed catalyst bed zones and is accommodated in two successive temperature zones A, B, and the transition from temperature zone A to temperature zone B does not spatially coincide with a transition from one fixed catalyst bed zone to another fixed catalyst bed zone.

[0001] The present invention relates to a process for partiallyoxidizing propene to acrolein in the gas phase under heterogeneouscatalysis by conducting a starting reaction gas mixture which comprisespropene, molecular oxygen and at least one inert gas, and contains themolecular oxygen and the propene in a molar O₂:C₃H₆ ratio of ≧1 in areaction stage over a fixed catalyst bed whose active composition is atleast one multimetal oxide comprising the elements Mo, Fe and Bi, insuch a way that

[0002] the fixed catalyst bed is arranged in two spatially successivetemperature zones A, B,

[0003] both the temperature of temperature zone A and the temperature oftemperature zone B are a temperature in the range from 290 to 380° C.,

[0004] the fixed catalyst bed consists of at least two spatiallysuccessive fixed catalyst bed zones, and the volume-specific activitywithin one fixed catalyst bed zone is substantially constant andincreases sharply in the flow direction of the reaction gas mixture atthe transition from one fixed catalyst bed zone to another fixedcatalyst bed zone,

[0005] the temperature zone A extends up to a conversion of the propeneof from 40 to 80 mol %,

[0006] on single pass of the starting reaction gas mixture through theentire fixed catalyst bed, the propene conversion is ≧90 moi% and theselectivity of acrolein formation based on converted propene is ≧90 mol%,

[0007]  the sequence in time in which the reaction gas mixture flowsthrough the temperature zones A, B corresponds to the alphabeticsequence of the temperature zones,

[0008] the hourly space velocity of the propene contained in thestarting reaction gas mixture on the fixed catalyst bed is ≧90 l (STP)of propene/l of fixed bed catalyst··h

[0009] the difference T^(maxA)−T^(maxB), formed from the highesttemperature T^(maxA) which the reaction gas mixture has withintemperature zone A, and the highest temperature T^(maxB) which thereaction gas mixture has within temperature zone B is ≧0° C.

[0010] The abovementioned process for partially oxidizing propene toacrolein under heterogeneous catalysis is known in principle, forexample, from WO 00/53556 and is of particular importance as a firstoxidation stage in the preparation of acrylic acid by two-stagecatalytic gas phase oxidation starting from propene. Acrylic acid is animportant monomer which finds use as such or in the form of its acrylicester for obtaining polymers suitable, for example, as adhesives.

[0011] In addition to molecular oxygen and the reactants, the startingreaction gas mixture contains inert gas in order to keep the reactiongas outside the explosion range, among other reasons.

[0012] One objective of such a catalyzed partial gas phase oxidation ofpropene to acrolein is to achieve a very high selectivity S^(AC) foracrolein (this is the number of moles of propene converted to acrolein,based on the number of moles of converted propene) on single pass of thereaction gas mixture through the reaction stage under otherwisepredefined boundary conditions.

[0013] A further objective is to achieve a very high conversion C^(P) ofpropylene (this is the number of moles of converted propene, based onthe number of moles of propene used) on single pass of the reaction gasmixture through the reaction stages under otherwise identical boundaryconditions.

[0014] When the S^(AC) is high at high C^(P), this corresponds to a highyield Y^(AC) of acrolein (Y^(Ac) is the product C^(P) S^(AC); i.e. thenumber of moles of propene converted to acrolein, based on the number ofmoles of propene used).

[0015] A further objective of such a heterogeneously catalyzed partialgas phase oxidation of propene to acrolein is to achieve a very highspace-time yield (STY^(AC)) of acrolein (in a continuous process, thisis the total amount of acrolein obtained per hour and unit volume of thefixed catalyst bed used in liters).

[0016] At a constant given yield Y^(AC), the greater the hourly spacevelocity of propene on the fixed catalyst bed of the reaction stage(this refers to the amount of propene in liters at STP (=l (STP); thevolume in liters which would be taken up by the appropriate amount ofpropene under standard conditions, i.e. at 25° C. and 1 bar) which isconducted as a constituent of the starting reaction gas mixture throughone liter of fixed catalyst bed per hour), the higher the space-timeyield.

[0017] DE-A 19948241, DE-A 19910506 and WO 0053556 teach that theabovementioned aims can indeed be achieved in principle by means of theprocess described at the outset for heterogeneously catalyzed partialgas phase oxidation of propene to acrolein.

[0018] This is advantageous in that a low volume-specific activity canbe achieved within a fixed catalyst bed or fixed catalyst bed zone, forexample, by diluting the actual shaped catalyst bodies bearing theactive composition with inert shaped diluent bodies free of activecomposition, which makes the fixed catalyst bed less expensive overall.

[0019] However, a disadvantage of the teachings of DE-A 19948241, ofDE-A 19910506 and of WO 00/53556 is that they have no appropriateimplementation example and thus leave open the specific configuration ofsuch a procedure.

[0020] In a similar manner, EP-A 1106598 teaches a process forheterogeneously catalyzed partial gas phase oxidation of propene toacrolein, in which the fixed catalyst bed of the reaction stage consistsof a plurality of spatially successive fixed catalyst bed zones whosevolume-specific activity is substantially constant within one fixedcatalyst bed zone and increases sharply in the flow direction of thereaction gas mixture at the transition from one fixed catalyst bed zoneto another fixed catalyst bed zone and may be arranged in a plurality oftemperature zones.

[0021] According to the teaching of EP-A 1106598, it is advantageouswhen fixed catalyst bed zones and temperature zones spatially coincide.

[0022] However, detailed in-house investigations have shown that spatialcoincidence of fixed catalyst bed zones and temperature zones isgenerally not beneficial for optimum achievement of the differentobjectives referred to.

[0023] Among other factors, this can be attributed to the transitionfrom temperature zone A to temperature zone B and the transition fromone fixed catalyst bed zone to another fixed catalyst bed zone beingtaken as substantially coactive or as substantially counteractivemeasures.

[0024] When they are taken at substantially coactive measures (forexample, transition from a colder temperature zone A to a hottertemperature zone B and transition from a fixed catalyst bed zone havinglower volume-specific activity to a fixed catalyst bed zone havinghigher volume-specific activity; both measures have the purpose ofsupporting the reaction), the overall effect achieved in the transitionregion frequently overshoots the aim pursued and this results, forexample, in a reduction of S^(AC).

[0025] When they are taken as substantially counteractive measures (forexample, transition from a hotter temperature zone A to a coldertemperature zone B and transition from a fixed catalyst bed zone havinglower volume-specific activity to a fixed catalyst bed zone havinghigher volume-specific activity; the first measure has the purpose ofreducing the reaction, the second measure has the purpose of supportingthe reaction), the effects frequently neutralize each other in thetransition region, resulting in a reduced C^(P).

[0026] It is an object of the present invention to provide a process forheterogeneously catalyzed partial gas phase oxidation of propene toacrolein in a multizone arrangement, which does not have thedisadvantages of the prior art.

[0027] We have found that this object is achieved by a process forpartially oxidizing propene to acrolein in the gas phase underheterogeneous catalysis by conducting a starting reaction gas mixturecomprising propene, molecular oxygen and at least one inert gas, andcontains the molecular oxygen and the propene in a molar O₂:C₃H₆ ratioof ≧1, in a reaction stage over a fixed catalyst bed whose activecomposition is at least one multimetal oxide comprising the elements Mo,Fe and Bi, in such a way that

[0028] the fixed catalyst bed is arranged in two spatially successivetemperature zones A, B,

[0029] both the temperature of temperature zone A and the temperature oftemperature zone B are a temperature in the range from 290 to 380° C.,

[0030] the fixed catalyst bed consists of at least two spatiallysuccessive fixed catalyst bed zones, and the volume-specific activitywithin one fixed catalyst bed zone is substantially constant andincreases sharply in the flow direction of the reaction gas mixture atthe transition from one fixed catalyst bed zone to another fixedcatalyst bed zone,

[0031] the temperature zone A extends up to a conversion of the propeneof from 40 to 80 mol %,

[0032] on single pass of the starting reaction gas mixture through theentire fixed catalyst bed, the propene conversion is ≧90 mol % and theselectivity of acrolein formation based on converted propene is ≧90 mol%,

[0033] the sequence in time in which the reaction gas mixture flowsthrough the temperature zones A, B corresponds to the alphabeticsequence of the temperature zones,

[0034] the hourly space velocity of the propene contained in thestarting reaction gas mixture on the fixed catalyst bed is ≧90 l (STP)of propene/l of fixed bed catalyst··h

[0035] the difference T^(maxA)−T^(maxB), formed from the highesttemperature T^(maxA) which the reaction gas mixture has withintemperature zone A, and the highest temperature T^(maxB) which thereaction gas mixture has within temperature zone B is ≧0° C.,

[0036] wherein the transition from temperature zone A to temperaturezone B in the fixed catalyst bed (spatially) does not coincide with atransition from one fixed catalyst bed zone to another fixed catalystbed zone.

[0037] In this document, the temperature of a temperature zone refers tothe temperature of the portion of the fixed catalyst bed disposed in thetemperature zone when practising the process according to the invention,except in the absence of a chemical reaction. When this temperature isnot constant within the temperature zone, the term temperature of atemperature zone then refers to the (numerical) mean of the temperatureof the fixed catalyst bed along the temperature zone. It is essentialthat the heating of the individual temperature zones is substantiallyindependent.

[0038] Since the heterogeneously catalyzed partial gas phase oxidationof propene to acrolein is a distinctly exothermic reaction, thetemperature of the reaction gas mixture on reactive pass through thefixed catalyst bed is generally different from the temperature of atemperature zone. They are normally above the temperature of thetemperature zone and generally pass through a maximum (heating pointmaximum) or fall starting from a maximum value within a temperaturezone.

[0039] In general, the difference T^(maxA)−T^(maxB) in the processaccording to the invention will not be more than 80° C. According to theinvention, T^(maxA)−T^(maxB) is preferably ≧3° C. and ≦70° C. With veryparticular preference, T^(maxA)−T^(maxB) in the process according to theinvention is ≧20° C. and ≦60° C.

[0040] When practising the process according to the invention in thecase of relatively low (≧90 l (STP)/l·h and ≦160 l (STP)/l·h) catalystvelocities of propene on the fixed catalyst bed, the T^(maxA)−T^(maxB)differences required according to the invention are normally attainedwhen, on the one hand, both the temperature of temperature zone A andthe temperature of temperature zone B are in the range from 290 to 380°C. and, on the other hand, the difference between the temperature oftemperature zone B (T_(B)) and the temperature of temperature zone A(T_(A)), i.e., T_(B)−T_(A), is ≦0° C. and ≧−20° C. or ≧−10° C. or ≦0° C.and ≧−5° C., or frequently ≦0° C. and ≧−3° C.

[0041] When practising the process according to the invention underincreased propene catalyst velocities (≧160 l (STP)/l·h and ≦300 l(STP)/l·h, or ≦600 l (STP)/l·h), the T^(maxA)−T^(maxB) differencesrequired according to the invention are normally attained when, on theone hand, both the temperature of temperature zone A and the temperatureof temperature zone B are in the range from 290 to 380° C. andT_(B)−T_(A) is ≧0° C. and ≦50° C., or ≧5° C. and ≦45° C, or ≧10° C. and≦40° C., or ≧15° C. and ≦30° C. or ≦35° C. (e.g. 20° C. or 25° C.).

[0042] The above statement regarding the T_(B)−T_(A) temperaturedifferences regularly also applies when the temperature of temperaturezone A is within the preferred range of from 305 to 365° C. or in theparticularly preferred range of from 310 to 340° C.

[0043] The hourly space velocity of propene on the fixed catalyst bed inthe process according to the invention may therefore be, for example,≧90 l (STP)/l·h and ≦300 l (STP)/l·h, or ≧110 l (STP)/l·h and ≦280 l(STP)/l·h or ≧130 l (STP)/l·h and ≦260 l (STP)/l·h, or ≧150 l (STP)/l·hand ≦240 l (STP)/l·h, or ≧170 l (STP)/l·h and ≦220 l (STP)/l·h, or ≧190l (STP)/l·h and ≦200 1 (STP)/l·h.

[0044] According to the invention, temperature zone A preferably extendsup to a propene conversion of from 50 to 70 mol % or from 60 to 70 mol%.

[0045] The working pressure in the process according to the inventioncan be either below atmospheric pressure (e.g. down to 0.5 bar) or aboveatmospheric pressure. Typically, the working pressure will be at valuesof from 1 to 5 bar, frequently from 1 to 3 bar. Normally, the reactionpressure will not exceed 100 bar.

[0046] In general, the propene conversion in the process according tothe invention based on a single pass will be ≧92 mol % or ≧94 mol % or≧96 mol %. The selectivity of acrolein formation will regularly be ≧92mol %, or ≧94 mol %, frequently ≧95 mol %, or ≧96 mol % or ≧97 mol %.

[0047] Useful sources for the molecular oxygen required are both air andair depleted of molecular nitrogen.

[0048] Useful catalysts for the fixed catalyst bed of the processaccording to the invention include all of those catalysts whose activecomposition is at least one multimetal oxide comprising Mo, Bi and Fe.

[0049] These are in particular the multimetal oxide active compositionsof the general formula I of DE-A 19955176, the multimetal oxide activecompositions of the general formula I of DE-A 19948523, the multimetaloxide active compositions of the general formulae I, II and III of DE-A10101695, the multimetal oxide active compositions of the generalformulae I, II and III of DE-A 19948248 and the multimetal oxide activecompositions of the general formulae I, II and III of DE-A 19955168 andalso the multimetal oxide active compositions specified in EP-A 700714.

[0050] Also suitable for the fixed catalyst bed are the multimetal oxidecatalysts comprising Mo, Bi and Fe which are disclosed in the documentsDE-A 10046957, DE-A 10063162, DE-C 3338380, DE-A 19902562, EP-A 15565,DE-C 2380765, EP-A 807465, EP-A 279374, DE-A 3300044, EP-A 575897, U.S.Pat. No. 4,438,217, DE-A 19855913, WO 98/24746, DE-A 19746210 (those ofthe general formula II), JP-A 91/294239, EP-A 293224 and EP-A 700714.This applies in particular for the exemplary embodiments in thesedocuments, and among these particular preference is given to those ofEP-A 15565, EP-A 575897, DE-A 19746210 and DE-A 19855913. Particularemphasis is given in this context to a catalyst according to example 1 cfrom EP-A 15565 and also to a catalyst to be prepared in a correspondingmanner but having the active compositionMo₁₂Ni_(6.5)Zn₂Fe₂Bi₁P_(0.0065)K_(0.06)Ox.10 SiO₂. Emphasis is alsogiven to the example having the serial number 3 from DE-A 19855913(stoichiometry: Mo₁₂Co₇Fe₃Bi_(0.6)K_(0.08)Si_(1.6)Ox) as an unsupportedhollow cylinder catalyst of geometry 5 mm×3 mm×2 mm or 5 mm×2 mm×2 mm(each external diameter×height×internal diameter) and also to theunsupported multimetal oxide II catalyst according to example 1 of DE-A19746210. Mention should also be made of the multimetal oxide catalystsof U.S. Pat. No. 4,438,217. The latter is true in particular when thesehave a hollow cylinder geometry of the dimensions 5.5 mm×3 mm×3.5 mm, or5 mm×2 mm×2 mm, or 5 mm×3 mm×2 mm, or 6 mm×3 mm×3 mm, or 7 mm×3 mm×4 mm(each external diameter×height×internal diameter). Likewise suitable arethe multimetal oxide catalysts and geometries of DE-A 10101695 or WO02/062737.

[0051] Also suitable are example 1 of DE-A 10046957 (stoichiometry:[Bi₂W₂O₉x 2WO₃]_(0,5).[Mo₁₂Co_(5.6)Fe_(2.94)Si_(1.59)K_(0.08)O_(x)]₁) asan unsupported hollow cylinder (ring) catalyst of geometry 5 mm×3 mm×2mm or 5 mm×2 mm×2 mm (each external diameter×length×internal diameter),and also the coated catalysts 1, 2 and 3 of DE-A 10063162(stoichiometry: Mo₁₂Bi_(1.0)Fe₃Co₇Si_(1.6)K_(0.08)), except as annularcoated catalysts of appropriate coating thickness and applied to supportrings of geometry 5 mm×3 mm×1.5 mm or 7 mm×3 mm×1.5 mm (each externaldiameter×length×internal diameter).

[0052] A multiplicity of the multimetal oxide active compositionssuitable for the catalysts of the fixed catalyst bed can be encompassedby the general formula I

Mo₁₂Bi_(a)Fe_(b)X¹ _(c)X² _(d)X³ _(e)X⁴ _(f)O_(n)  (I)

[0053] where the variables are defined as follows:

[0054] X¹=nickel and/or cobalt,

[0055] X²=thallium, an alkali metal and/or an alkaline earth metal,

[0056] X³=zinc, phosphorus, arsenic, boron, antimony, tin, cerium, leadand/or tungsten,

[0057] X⁴=silicon, aluminum, titanium and/or zirconium,

[0058] a=from 0.5 to 5,

[0059] b=from 0.01 to 5, preferably from 2 to 4,

[0060] c=from 0 to 10, preferably from 3 to 10,

[0061] d=from 0 to 2, preferably from 0.02 to 2,

[0062] e=from 0 to 8, preferably from 0 to 5,

[0063] f=from 0 to 10 and

[0064] n=a number which is determined by the valency and frequency ofthe elements in I other than oxygen.

[0065] They are obtainable in a manner known per se (see, for example,DE-A 4023239) and are customarily shaped undiluted to give spheres,rings or cylinders or else used in the form of coated catalysts, i.e.preshaped inert support bodies coated with the active composition. Itwill be appreciated that they may also be used as catalysts in powderform.

[0066] In principle, active compositions of the general formula I can beprepared in a simple manner by obtaining a very intimate, preferablyfinely divided dry mixture corresponding to their stoichiometry fromsuitable sources of their elemental constituents and calcining it attemperatures of from 350 to 650° C. The calcination may be effectedeither under inert gas or under an oxygen atmosphere, for example air(mixture of inert gas and oxygen) and also under a reducing atmosphere(for example mixture of inert gas, NH₃, CO and/or H₂). The calcinationtime can be from a few minutes to a few hours and typically decreaseswith temperature. Useful sources for the elemental constituents of themultimetal oxide active compositions I are those compounds which arealready oxides and/or those compounds which can be converted to oxidesby heating, at least in the presence of oxygen.

[0067] In addition to the oxides, such useful starting compounds includein particular halides, nitrates, formates, oxalates, citrates, acetates,carbonates, amine complexes, ammonium salts and/or hydroxides (compoundssuch as NH₄OH, (NH₄)₂CO₃, NH₄NO₃, NH₄CHO₂, CH₃COOH, NH₄CH₃CO₂ and/orammonium oxalate, which decompose and/or can be decomposed on latercalcining at the latest to release compounds in gaseous form can beadditionally incorporated into the intimate dry mixture).

[0068] The starting compounds for preparing the multimetal oxide activecompositions I can be intimately mixed in dry or in wet form. When theyare mixed in dry form, the starting compounds are advantageously used asfinely divided powders and subjected to calcination after mixing andoptional compacting. However, preference is given to intimate mixing inwet form. Customarily, the starting compounds are mixed with each otherin the form of an aqueous solution and/or suspension. Particularlyintimate dry mixtures are obtained in the mixing process described whenthe starting materials are exclusively sources of the elementalconstituents in dissolved form. The solvent used is preferably water.Subsequently, the aqueous composition obtained is dried, and the dryingprocess is preferably effected by spray-drying the aqueous mixture atexit temperatures of from 100 to 150° C.

[0069] Typically, the multimetal oxide active compositions of thegeneral formula I are used in the fixed catalyst bed not in powder form,but rather shaped into certain catalyst geometries, and the shaping maybe effected either before or after the final calcination. For example,unsupported catalysts can be prepared from the powder form of the activecomposition or its uncalcined and/or partially calcined precursorcomposition by compacting to the desired catalyst geometry (for exampleby tableting or extruding), optionally with the addition of assistants,for example graphite or stearic acid as lubricants and/or shapingassistants and reinforcing agents such as microfibers of glass,asbestos, silicon carbide or potassium titanate. Examples of unsupportedcatalyst geometries include solid cylinders or hollow cylinders havingan external diameter and a length of from 2 to 10 mm. In the case of thehollow cylinder, a wall thickness of from 1 to 3 mm is advantageous. Itwill be appreciated that the unsupported catalyst can also havespherical geometry, and the spherical diameter can be from 2 to 10 mm.

[0070] A particularly advantageous hollow cylinder geometry is 5 mm×3mm×2 mm (external diameter×length×internal diameter), in particular inthe case of unsupported catalysts.

[0071] It will be appreciated that the pulverulent active composition orits pulverulent precursor composition which is yet to be calcined and/orpartially calcined can be shaped by applying to preshaped inert catalystsupports. The coating of the support bodies to produce the coatedcatalysts is generally performed in a suitable rotatable vessel, asdisclosed, for example, by DE-A 2909671, EP-A 293859 or EP-A 714700. Tocoat the support bodies, the powder composition to be applied isadvantageously moistened and dried again after application, for exampleby means of hot air. The coating thickness of the powder compositionapplied to the support body is advantageously selected within the rangefrom 10 to 1 000 μm, preferably within the range from 50 to 500 μm andmore preferably within the range from 150 to 250 μm.

[0072] Useful support materials are the customary porous or nonporousaluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide,silicon carbide or silicates such as magnesium silicate or aluminumsilicate. They generally behave inertly with regard to the targetreaction on which the process according to the invention in the firstreaction stage is based. The support bodies may have a regular orirregular shape, although preference is given to regularly shapedsupport bodies having a distinct surface roughness, for example spheresor hollow cylinders. It is suitable to use substantially nonporous,surface-roughened spherical supports made of steatite (e.g. SteatiteC220 from CeramTec) whose diameter is from 1 to 8 mm, preferably from 4to 5 mm. However, suitable support bodies also include cylinders whoselength is from 2 to 10 mm and whose external diameter is from 4 to 10mm. In the case of rings suitable as support bodies according to theinvention, the wall thickness is also typically from 1 to 4 mm.According to the invention, annular support bodies to be used preferablyhave a length of from 2 to 6 mm, an external diameter of from 4 to 8 mmand a wall thickness of from 1 to 2 mm. Suitable as support bodiesaccording to the invention are in particular rings of geometry 7 mm×3mm×4 mm (external diameter×length×internal diameter). It will beappreciated that the fineness of the catalytically active oxidecompositions to be applied to the surface of the support body will beadapted to the desired coating thickness (cf. EP-A 714 700).

[0073] Suitable multimetal oxide active compositions for the catalystsof the reaction stage are also compositions of the general formula II

[Y¹ _(a′)Y² _(b′)O_(x′)]_(p)[Y³ _(c′)Y⁴ _(d′)Y⁵ _(e′)Y⁶ _(f′)Y⁷ _(g′)Y²_(h′)O_(y′)]_(q)  (II)

[0074] where the variables are defined as follows:

[0075] Y¹=only bismuth or bismuth and at least one of the elementstellurium, antimony, tin and copper,

[0076] Y²=molybdenum or molybdenum and tungsten,

[0077] Y³=an alkali metal, thallium and/or samarium,

[0078] Y⁴=an alkaline earth metal, nickel, cobalt, copper, manganese,zinc, tin, cadmium and/or mercury,

[0079] Y⁵=iron or iron and at least one of the elements chromium andcerium,

[0080] Y⁶=phosphorus, arsenic, boron and/or antimony,

[0081] Y⁷=a rare earth metal, titanium, zirconium, niobium, tantalum,rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium,silicon, germanium, lead, thorium and/or uranium,

[0082] a′=from 0.01 to 8,

[0083] b′=from 0.1 to 30,

[0084] c′=from 0 to 4,

[0085] d′=from 0 to 20,

[0086] e′=from 0 to 20,

[0087] f′=from 0 to 6,

[0088] g′=from 0 to 15,

[0089] h′=from 8 to 16,

[0090] x′,y′=numbers which are determined by the valency and frequencyof the elements in II other than oxygen and

[0091] p,q=numbers whose p/q ratio is from 0.1 to 10,

[0092] comprising three-dimensional regions of the chemical compositionY¹ _(a′) Y² _(b′)O_(x′)which are delimited from their local environmentas a consequence of their different chemical composition from theirlocal environment, and whose maximum diameter (longest line cuttingthrough the center of the region and connecting two points on thesurface (interface) of the region) is from 1 nm to 100 μm, frequentlyfrom 10 nm to 500 nm or from 1 μm to 50 or 25 μm.

[0093] Particularly advantageous multimetal oxide compositions IIaccording to the invention are those in which Y¹ is only bismuth.

[0094] Among these, preference is given in turn to those of the generalformula III

[Bi_(a″)Z² _(b″)O_(x″)]_(p″)[Z² ₁₂Z³ _(c″)Z⁴ _(d″)Fe_(e″)Z⁵ _(f″)Z⁶_(g″)Z⁷ _(h″)O_(y″)]_(g″)  (III)

[0095] where the variables are defined as follows:

[0096] Z²=molybdenum or molybdenum and tungsten,

[0097] Z³=nickel and/or cobalt,

[0098] Z⁴=thallium, an alkali metal and/or an alkaline earth metal,

[0099] Z⁵=phosphorus, arsenic, boron, antimony, tin, cerium and/or lead,

[0100] Z⁶=silicon, aluminum, titanium and/or zirconium,

[0101] Z⁷=copper, silver and/or gold,

[0102] a″=from 0.1 to 1,

[0103] b″=from 0.2 to 2,

[0104] c″=from 3 to 10,

[0105] d″=from 0.02 to 2,

[0106] e″=from 0.01 to 5, preferably from 0.1 to 3,

[0107] f″=from 0 to 5,

[0108] g″=from 0 to 10,

[0109] h″=from 0 to 1,

[0110] x″,y″=numbers which are determined by the valency and frequencyof the elements in III other than oxygen,

[0111] p″,q″=numbers whose p″/q″ ratio is from 0.1 to 5, preferably from0.5 to 2,

[0112] and particular preference is given to those compositions III inwhich Z² _(b″)=(tungsten)_(b″) and Z² ₁₂=(molybdenum)₁₂.

[0113] It is also advantageous when at least 25 mol % (preferably atleast 50 mol % and more preferably at least 100 mol %) of the totalproportion of [Y¹ _(a′)Y² _(b′)O_(x′)]_(p)([Bi_(a″)Z² _(b″)O_(x″)]_(p″))of the multimetal oxide compositions II (multimetal oxide compositionsII) suitable according to the invention in the multimetal oxidecompositions II (multimetal oxide compositions III) suitable accordingto the invention are in the form of three-dimensional regions of thechemical composition Y¹ _(a′)Y² _(b′)[Bi_(a″)Z² _(b″)O_(x″)] which aredelimited from their local environment as a consequence of theirdifferent chemical composition from their local environment, and whosemaximum diameter is in the range from 1 nm to 100 μm.

[0114] With regard to the shaping, the statements made for themultimetal oxide I catalysts apply to the multimetal oxide II catalysts.The preparation of multimetal oxide II active compositions is described,for example, in EP-A 575897 and also in DE-A 19855913.

[0115] It is essential to the invention that the fixed catalyst bedconsists of at least two spatially successive fixed catalyst bed zones,and the volume-specific activity within one fixed catalyst bed zone issubstantially constant and increases sharply in the flow direction ofthe reaction gas mixture at the transition from one fixed catalyst bedzone to another fixed catalyst bed zone.

[0116] The volume-specific (i.e. normalized to the unit of theparticular bed volume) activity of a fixed bed catalyst zone can then beadjusted over the fixed catalyst bed zone in a substantially constantmanner by starting from a basic amount of shaped catalyst bodiesprepared in a uniform manner (their bed corresponds to the maximumachievable volume-specific activity) and homogeneously diluting it inthe particular fixed catalyst bed zone with shaped bodies (shapeddiluent bodies) which behave substantially inertly with regard to theheterogeneously catalyzed partial gas phase oxidation. The higher theproportion of shaped diluent bodies selected, the less activecomposition and catalyst activity, present in a certain volume of thebed. Useful materials for such inert shaped diluent bodies are inprinciple all of those which are suitable as support material for coatedcatalysts suitable according to the invention.

[0117] Useful such materials include, for example, porous or nonporousaluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide,silicon carbide, silicates such as magnesium silicate or aluminumsilicate or the steatite already mentioned above (e.g. Steatite C-220from CeramTec).

[0118] The geometry of such inert shaped diluent bodies may in principlebe as desired. In other words, they may, for example, be spheres,polygons, solid cylinders or else rings. According to the invention, theinert shaped diluent bodies selected will preferably be those whosegeometry corresponds to that of the shaped catalyst bodies to be dilutedwith them.

[0119] According to the invention, it is advantageous when the chemicalcomposition of the active composition used does not vary over the entirefixed catalyst bed. In other words, although the active composition usedfor a single shaped catalyst body can be a mixture of differentmultimetal oxides comprising the elements Mo, Fe and Bi, the samemixture then has to be used for all shaped catalyst bodies of the fixedcatalyst bed.

[0120] A volume-specific activity increasing zone by zone over the fixedcatalyst bed in the flow direction of the reaction gas mixture cantherefore be achieved for the process according to the invention in asimple manner, for example, by beginning the bed in a first fixedcatalyst bed zone with a high proportion of inert shaped diluent bodiesbased on one type of shaped catalyst bodies, and then reducing thisproportion of shaped diluent bodies zone by zone in the flow direction.

[0121] However, a zone by zone increase in the volume-specific activityis also possible, for example, by increasing the thickness of the activecomposition layer applied to the support zone by zone at constantgeometry and active composition type of a coated shaped catalyst bodyor, in a mixture of coated catalysts having the same geometry but havingdifferent proportions by weight of the active composition by increasingthe proportion of shaped catalyst bodies having higher proportions byweight of active composition zone by zone. Alternatively, the activecompositions themselves can be diluted in the course of the activecomposition preparation by, for example, incorporating inert, dilutingmaterials such as hard-fired silica into the dry mixture of startingcompounds to be calcined. Different amounts of diluting material addedlead automatically to different activities. The more diluting materialis added, the lower the resulting activity will be. A similar effect canalso be achieved, for example, in the case of mixtures of unsupportedcatalysts and of coated catalysts (having identical active composition)by varying the mixing ratio in an appropriate manner. A variation in thevolume-specific activity can also be achieved by the use of catalystgeometries having different bed densities (for example, in the case ofunsupported catalysts having identical active compositions of thedifferent geometries). It will be appreciated that the variantsdescribed can also be used in combination.

[0122] It is of course also possible to use mixtures of catalysts havingchemically different active compositions and, as a consequence of thesedifferent compositions, having different activities for the fixedcatalyst bed. These mixtures may in turn, zone by zone, be varied intheir composition and/or be diluted with different amounts of inertshaped diluent bodies.

[0123] Upstream and/or downstream of the fixed catalyst bed may bedisposed beds consisting exclusively of inert material (for example onlyshaped diluent bodies) (in this document, they are not included forterminology purposes in the fixed catalyst bed, since they contain noshaped bodies which have multimetal oxide active composition). Theshaped diluent bodies used for the inert bed can have the same geometryas the shaped catalyst bodies used in the fixed catalyst bed. However,the geometry of the shaped diluent bodies used for the inert bed mayalso be different to the abovementioned geometry of the shaped catalystbodies (for example, spherical instead of annular).

[0124] Frequently, the shaped bodies used for such inert beds have theannular geometry 7 mm×7 mm×4 mm (external diameter×length×internaldiameter) or the spherical geometry having a diameter d=4-5 mm.Temperature zones A and B in the process according to the invention mayalso extend to the inert beds. According to the invention, it isadvantageous when neither temperature zone A nor temperature zone Bcovers more than three fixed catalyst bed zones (according to theinvention, at least one fixed catalyst bed zone is necessarily coveredby both temperature zones).

[0125] According to the invention, it is particularly advantageous whenthe entire fixed catalyst bed comprises not more than five,advantageously not more than four or three fixed catalyst bed zones.

[0126] According to the invention, at the transition from one fixedcatalyst bed zone to another fixed catalyst bed zone (in the flowdirection of the reaction gas mixture), the volume-specific activecomposition (i.e. the weight of the multimetal oxide composition presentin the uniform bed volume) should (in the case of uniform activecomposition over the entire fixed catalyst bed) advantageously increaseby at least 5% by weight, preferably by at least 10% by weight (thisapplies in particular in the case of uniform shaped catalyst bodies overthe entire fixed catalyst bed). In general, this increase in the processaccording to the invention will not be more than 50% by weight, usuallynot more than 40% by weight. According to the invention, in the case ofuniform active composition over the entire fixed catalyst bed, thedifference in the volume-specific active composition of the fixedcatalyst bed zone having the lowest volume-specific activity and thefixed catalyst bed zone having the highest volume-specific activityshould also advantageously not be more than 50% by weight, preferablynot more than 40% by weight, and generally not more than 30% by weight.

[0127] In the process according to the invention, the fixed catalyst bedwill frequently consist of only two fixed catalyst bed zones.

[0128] According to the invention, preference is given to the last fixedcatalyst bed zone in the flow direction of the reaction gas mixturebeing undiluted. In other words, it preferably consists exclusively ofshaped catalyst bodies. If required, it may also consist of a bed ofshaped catalyst bodies whose volume-specific activity is reduced, forexample by dilution with inert material, for example by 10%.

[0129] When the fixed catalyst bed consists of only two fixed catalystbed zones, it is generally advantageous according to the invention (asis quite generally the case in the process according to the invention)when the fixed catalyst bed zone having the highest volume-specificactivity does not extend into temperature zone A (in particular when theheating is effected in temperature zone A and temperature zone B bymeans of a flowing heat carrier which in each case flows incountercurrent to the reaction gas mixture). In other words, the fixedcatalyst bed zone having the lower volume-specific activity will extendinto temperature zone B and the fixed catalyst bed zone having thehigher volume-specific activity will begin and end in temperature zone B(i.e. have its beginning beyond the transition from temperature zone Ato temperature zone B).

[0130] When the fixed catalyst bed consists of only three fixed catalystbed zones, it is generally advantageous according to the invention whenthe fixed catalyst bed zone having the highest volume-specific activitydoes not extend into temperature zone A but begins and ends intemperature zone B, i.e. has its beginning beyond the transition fromtemperature zone A to temperature zone B (in particular when the heatingin temperature zone A and temperature zone B is effected by means of aflowing heat carrier which in each case flows in countercurrent to thereaction gas mixture). In other words, the fixed catalyst bed zonehaving the second highest volume-specific activity will normally extendboth into temperature zone A and temperature zone B.

[0131] When the fixed catalyst bed consists of four fixed catalyst bedzones, it is generally advantageous in accordance with the inventionwhen the fixed catalyst bed zone having the third highestvolume-specific activity extends both into temperature zone A and intotemperature zone B (in particular when temperature zone A andtemperature zone B are heated by means of a flowing heat carrier whichin each case flows in countercurrent to the reaction gas mixture).

[0132] In the case of cocurrent flow of reaction gas mixture and heatcarriers in temperature zones A and B, it may be advantageous in theprocess according to the invention if the fixed catalyst bed zone havingthe highest volume-specific activity extends into temperature zone A.

[0133] Generally, the volume-specific activity between two fixedcatalyst bed zones can be differentiated experimentally in a simplemanner by passing the same propene-containing starting reaction gasmixture over fixed catalyst beds of the same length but eachcorresponding to the composition of the particular fixed catalyst bedzone under identical boundary conditions (preferably the conditions ofthe contemplated process). The higher amount of propene convertedindicates the higher volume-specific activity.

[0134] When the total length of the fixed catalyst bed is L¹, it isadvantageous in accordance with the invention if there is no transitionfrom one fixed catalyst bed zone to another fixed catalyst bed zonewithin the region of $X \pm {L \cdot \frac{4}{100}}$

[0135] or within the region of $X \pm {L \cdot \frac{3}{100}}$

[0136] or within the region of $X \pm {L \cdot \frac{2}{100}}$

[0137] where X is the location (the position) within the fixed catalystbed of the transition from temperature zone A to temperature zone B.

[0138] Preference is given in accordance with the invention to the fixedcatalyst bed in the process according to the invention being structuredas follows in the flow direction of the reaction gas mixture.

[0139] First, to a length of from 10 to 60%, preferably from 10 to 50%,more preferably from 20 to 40% and most preferably from 25 to 35% (i.e.,for example, to a length of from 0.70 to 1.50 m, preferably from 0.90 to1.20 m), each of the total length of the fixed catalyst bed 1, ahomogeneous mixture of shaped catalyst bodies and shaped diluent bodies(both preferably having substantially the same geometry), in which theproportion by weight of the shaped diluent bodies (the densities ofshaped catalyst bodies and of shaped diluent bodies generally differonly slightly) is normally from 5 to 40% by weight, or from 10 to 40% byweight, or from 20 to 40% by weight, or from 25 to 35% by weight.According to the invention, this first zone of the fixed catalyst bed isadvantageously followed up to the end of the length of the fixedcatalyst bed (i.e., for example, to a length of from 2.00 to 3.00 m,preferably from 2.50 to 3.00 m) either by a bed of shaped catalystbodies diluted only to a slighter extent (than in the first zone), or,most preferably, an unaccompanied (undiluted) bed of the same shapedcatalyst bodies which have also been used in the first zone. Theaforesaid applies in particular when the shaped catalyst bodies used inthe fixed catalyst bed are unsupported catalyst rings or coated catalystrings (in particular those which are specified as preferred in thisdocument). For the purposes of the abovementioned structuring, both theshaped catalyst bodies and the shaped diluent bodies in the processaccording to the invention advantageously have substantially the ringgeometry 5 mm×3 mm×2 mm (external diameter×length×internal diameter).

[0140] The aforesaid also applies when, instead of inert shaped diluentbodies, shaped coated catalyst bodies are used whose active compositioncontent is from 2 to 15% by weight lower than the active compositioncontent of any shaped coated catalyst bodies used at the end of thefixed catalyst bed.

[0141] A pure inert material bed whose length, based on the length ofthe fixed catalyst bed, is advantageously from 5 to 20% generallyprecedes the fixed catalyst bed in the flow direction of the reactiongas mixture. It is normally utilized as a heating zone for the reactiongas mixture.

[0142] According to the invention, the fixed catalyst bed zone havingthe lower volume-specific activity in the aforementioned fixed catalystbed then advantageously extends for from 5 to 20%, frequently from 5 to15%, of its length into temperature zone B.

[0143] According to the invention, temperature zone A alsoadvantageously extends to a preliminary bed of inert material which isoptionally used.

[0144] In an advantageous manner from an application point of view, thereaction stage according to the invention of the process according tothe invention is carried out in a two-zone tube bundle reactor, asdescribed, for example, in DE-A 19910508, 19948523, 19910506 and19948241. A preferred variant of a two-zone tube bundle reactor whichcan be used in accordance with the invention is disclosed by DE-C2830765. However, the two-zone tube bundle reactors disclosed in DE-C2513405, U.S. Pat. No. 3,147,084, DE-A 2201528, EP-A 383224 and DE-A2903218 are also suitable for carrying out the first reaction stage ofthe process according to the invention.

[0145] In other words, in the simplest manner, the fixed catalyst bed tobe used in accordance with the invention (possibly with downstreamand/or upstream inert beds) is disposed in the metal tubes of a tubebundle reactor and two spatially separated heating media, generally saltmelts, are conducted around the metal tubes. The tube section over whichthe particular salt bath extends represents a temperature zone inaccordance with the invention. In other words, in the simplest manner,for example, a salt bath A flows around that section of the tubes(temperature zone A) in which propene is oxidatively converted (onsingle pass) until a conversion in the range from 40 to 80 mol % isachieved, and a salt bath B flows around the section of the tubes(reaction zone B) in which the propene is subsequently oxidativelyconverted (on single pass) until a conversion value of at least 90 mol %is achieved (if required, the temperature zones A, B to be used inaccordance with the invention can be followed by further reaction zoneswhich are maintained at individual temperatures).

[0146] It is advantageous from an application point of view if thereaction stage of the process according to the invention includes nofurther temperature zones. In other words, the salt bath Badvantageously flows around the section of the tubes in which propene issubsequently oxidatively converted (on single pass) up to a conversionvalue of ≧90 mol %, or ≧92 mol %, or ≧94 mol % or more.

[0147] Typically, the beginning of the temperature zone B lies beyondthe heating point maximum of temperature zone A.

[0148] According to the invention, both salt baths A, B can be conductedin cocurrent or in countercurrent through the space surrounding thereaction tubes relative to the flow direction of the reaction gasmixture flowing through the reaction tubes. It will be appreciated that,in accordance with the invention, cocurrent flow may be applied intemperature zone A and countercurrent flow in temperature zone B (orvice versa).

[0149] In all of the aforementioned cases, it will be appreciated that atransverse flow can be superimposed on the parallel flow of the saltmelt relative to the reaction tubes taking place within the particulartemperature zone, so that the individual temperature zone corresponds toa tube bundle reactor as described in EP-A 700714 or in EP-A 700893,which results overall in a meandering flow profile of the heat exchangemedium in a longitudinal section through the catalyst tube bundle.

[0150] Advantageously, the starting reaction gas mixture in the processaccording to the invention is fed to the fixed catalyst bed preheated tothe reaction temperature.

[0151] Typically, the catalyst tubes in the two-zone tube bundlereactors are manufactured from ferritic steel and typically have a wallthickness of from 1 to 3 mm. Their internal diameter is generally from20 to 30 mm, frequently from 21 to 26 mm. Their length is advantageouslyfrom 2 to 4 m, preferably from 2.5 to 3.5 m. In each temperature zone,the fixed catalyst bed occupies at least 60%, or at least 75%, or atleast 90%, of the length of the zone. Any remaining length is optionallyoccupied by an inert bed. It is advantageous from an application pointof view for the number of catalyst tubes accommodated in the tube bundlevessel to be at least 5 000, preferably at least 10 000. Frequently, thenumber of catalyst tubes accommodated in the reaction vessel is from 15000 to 30 000. Tube bundle reactors having a number of catalyst tubesabove 40 000 are usually exceptional. Within the vessel, the catalysttubes are normally homogeneously distributed (preferably 6 equidistantadjacent tubes per catalyst tube), and the distribution isadvantageously selected in such a way that the separation of the centralinternal axes of immediately adjacent catalyst tubes (the catalyst tubepitch) is from 35 to 45 mm (cf., for example, EP-B 468290).

[0152] Useful heat exchange media for the two-zone method are also inparticular fluid heating media. It is particularly advantageous to usemelts of salts such as potassium nitrate, potassium nitrite, sodiumnitrite and/or sodium nitrate, or of low-melting metals such as sodium,mercury and also alloys of different metals.

[0153] In general, in all of the aforementioned flow arrangements in thetwo-zone tube bundle reactors, the flow rate within the two heatexchange medium circuits required is selected in such a way that thetemperature of the heat exchange medium rises from the entrance into thetemperature zone to the exit from the temperature zone (as a result ofthe exothermicity of the reaction) by from 0 to 15° C. In other words,the aforementioned ΔT may be from 1 to 10° C., or from 2 to 8° C., orfrom 3 to 6° C., in accordance with the invention.

[0154] According to the invention, the entrance temperature of the heatexchange medium into temperature zone A is normally in the range from290 to 380° C., preferably in the range from 305 to 365° C. and morepreferably in the range from 310 to 340° C. or to 330° C. According tothe invention, in the case of propene catalyst velocities on the fixedcatalyst bed of ≧90 l (STP)/l·h and <160 l (STP)/l·h, the entrancetemperature of the heat exchange medium into temperature zone B islikewise in the range from 290 to 380° C., but at the same timenormally, advantageously in accordance with the invention, from ≧0° C.to ≦20° C., or ≦10° C., or ≧0° C. and ≦5° C., or frequently ≧0° C. and≦3° C., below the entrance temperature of the heat exchange mediumentering temperature zone A. According to the invention, in the case ofpropene catalyst velocities on the fixed catalyst bed of ≧160 l(STP)/l·h and (generally) ≦300 l (STP)/l·h (or 600 l (STP)/l·h), theentrance temperature of the heat exchange medium into temperature zone Bwill likewise be in the range from 290 to 380° C., but normally,advantageously in accordance with the invention, from ≧0° C. to ≦50° C.,or ≧5° C. and ≦45° C., or ≧10° C. and ≦40° C., or ≧15° C. and ≦30° C. or≦35° C. (for example 20° C. or 25° C.), above the entrance temperatureof the heat exchange medium entering temperature zone A.

[0155] It is pointed out once again at this juncture that, for animplementation of the reaction stage of the process according to theinvention, it is possible to use in particular the two-zone tube bundlereactor type described in DE-B 2201528 which includes the possibility ofremoving the hotter heat exchange medium of temperature zone B totemperature zone A, in order to optionally heat a cold starting reactiongas mixture or a cold cycle gas. The tube bundle characteristics withinan individual temperature zone may also be configured as described inEP-A 382098.

[0156] The acrolein can be removed in a manner known per se from theproduct gas mixture leaving the reaction stage and the remaining gas canbe partly recycled in a manner known per se as diluent gas into thepropene partial oxidation. However, it will advantageously be used tochange a downstream heterogeneously catalyzed partial oxidation of theacrolein contained therein to acrylic acid. Advantageously, it will becooled in a direct and/or indirect manner before entry into such adownstream reaction stage, in order to thus suppress subsequent fullcombustion of portions of the acrolein formed in the propene oxidationstage.

[0157] An aftercooler is usually connected in between the two reactionstages. In the simplest case, said aftercooler may be an indirect tubebundle heat exchanger. Subsequently, oxygen is usually supplied in theform of air, in order to provide the superstoichiometric oxygen contentwhich is advantageous for such an acrolein oxidation. Such an acroleinpartial oxidation is advantageously carried out in a manner similar tothe process of the invention.

[0158] The propene content in the starting reaction gas mixture in theprocess according to the invention can, for example, be at values offrom 4 to 15% by volume, frequently from 5 to 12% by volume, or from 5to 8% by volume (based in each case on the total volume).

[0159] Frequently, the process according to the invention will becarried out as a propene:oxygen:inert gases (including steam) volumeratio in the starting reaction gas mixture 1 of 1:(1.0 to 3.0):(5 to25), preferably 1:(1.7 to 2.3):(10 to 15). In general, at least 20% ofthe volume of the inert gas will consist of molecular nitrogen. However,≧30% by volume, or ≧40% by volume, or ≧50% by volume, or ≧60% by volume,or ≧70% by volume, or ≧80% by volume, or ≧90% by volume, or ≧95% byvolume, of the inert gas may consist of molecular nitrogen (in thisdocument, inert diluent gases generally refer to those of which lessthan 5%, preferably less than 2%, is converted on single pass throughthe reaction stage; in addition to molecular nitrogen, these are, forexample, gases such as propane, ethane, methane, pentane, butane, CO₂,CO, steam and/or noble gases). Up to 50 mol %, or up to 75 mol % andmore of the inert diluent gas in the process according to the inventioncan consist of propane. Cycle gas, as remains after the removal of theacrolein and/or acrylic acid from the product gas mixture (for examplein the case of a downstream second oxidation stage), can also be aconstituent of the diluent gas.

[0160] The starting reaction gas mixtures which are advantageousaccording to the invention are, for example, those which are composed offrom 6 to 15 (preferably from 7 to 11) % by volume of propene, from 4 to20 (preferably from 6 to 12) % by volume of water, from ≧0 to 10(preferably from ≧0 to 5) of constituents other % by volume thanpropene, water, oxygen and nitrogen,

[0161] sufficient molar oxygen that the molar ratio of molar oxygenpresent to propene present is from 1.5 to 2.5 (preferably from 1.6 to2.2), and the remainder up to 100% by volume of the total amount ofmolecular nitrogen.

[0162] as recommended by DE-A 10302715.

[0163] It is emphasized at this juncture that the multimetal oxidecompositions of DE-A 10261186 are advantageous as active compositons forthe fixed catalyst bed.

[0164] Designs of a two-zone tube bundle reactor for the reaction stageaccording to the invention which are advantageous in accordance with theinvention can have the following construction (the detailedconfiguration of the construction can be as described in the utilitymodel applications 202 19 277.6, 2002 19 278.4 and 202 19 279.2 or inthe PCT applications PCT/EP02/14187, PCT/EP02/14188 or PCT/EP02/14189):

[0165] Catalyst Tubes: material of the catalyst tubes: ferritic steel;dimensions of the catalyst tubes: length, for example, 3 500 mm;external diameter, for example, 30 mm; wall thickness, for example, 2mm;

[0166] number of catalyst tubes in the tube bundle: for example, 30 000,or 28 000, or 32 000, or 34 000; in addition up to 10 thermal tubes (asdescribed in EP-A 873 783 and EP-A 12 70 065) which are charged in thesame way as the catalyst tubes (in a spiral manner rotating from thevery outside toward the inside), for example of the same length and wallthickness but having an external diameter of, for example, 33.4 mm and acentered thermowell of external diameter, for example, 8 mm and wallthickness of, for example, 1 mm;

[0167] reactor (same material as the catalyst tubes):

[0168] cylindrical vessel of internal diameter 6 000-8 000 mm;

[0169] reactor hoods plated with stainless steel of the type 1.4541;plating thickness: a few mm;

[0170] annularly arranged tube bundle, for example with free centralspace:

[0171] diameter of the free central space: for example, 1 000-2 500 mm(for example 1 200 mm, or 1 400 mm, or 1 600 mm, or 1 800 mm, or 2 000mm, or 2 200 mm, or 2 400 mm);

[0172] normally homogeneous catalyst tube pitch in the tube bundle (6equidistant adjacent tubes per catalyst tube), arrangement in anequilateral triangle, catalyst tube pitch (separation of the centralinternal axes of immediately adjacent catalyst tubes): 35-45 mm, forexample 36 mm, or 38 mm, or 40 mm, or 42 mm, or 44 mm;

[0173] the catalyst tubes are secured and sealed by their ends incatalyst tube plates (upper plate and lower plate each having athickness, for example, of 100-200 mm) and open at their upper ends intoa hood joined to the vessel which has an inlet for the starting reactiongas mixture; a separating plate of thickness 20-100 mm disposed, forexample, at half the catalyst tube length, divides the reactor spacesymmetrically into two temperature zones A (upper zone) and B (lowerzone); each temperature zone is divided into 2 equidistant longitudinalsections by deflecting plates;

[0174] the deflecting plate preferably has annular geometry; thecatalyst tubes are advantageously secured and sealed at the separatingplate; they are not secured and sealed at the deflecting plates, so thatthe transverse flow rate of the salt melt within one zone is veryconstant;

[0175] each zone is provided with salt melt as a heat carrier by its ownsalt pump; the feed of the salt melt is, for example, below thedeflecting plate and the withdrawal is, for example, above thedeflecting plate;

[0176] a substream is, for example, removed from both salt melt circuitsand cooled, for example, in one common or two separate indirect heatexchangers (steam generation);

[0177] in the first case, the cooled salt melt stream is divided,combined with the particular residual stream and pressurized into thereactor by the particular pump into the appropriate annular channelwhich divides the salt melt over the circumference of the vessel;

[0178] the salt melt reaches the tube bundle through the window disposedin the reactor jacket; the flow is, for example, in a radial directionto the tube bundle;

[0179] in each zone, the salt melt flows around the catalyst tubes asdictated by the deflection plate, for example in the sequence

[0180] from the outside inward,

[0181] from the inside outward;

[0182] the salt melt flows through a window mounted around thecircumference of the vessel and collects at the end of each zone in anannular channel disposed around the reactor jacket, in order to bepumped in a circuit including substream cooling;

[0183] the salt melt is conducted from bottom to top through eachtemperature zone.

[0184] The reaction gas mixture leaves the reactor of the reaction stageaccording to the invention at a temperature a few degrees higher thanthe salt bath entrance temperature. For further processing in adownstream acrolein oxidation stage, the reaction gas mixture isadvantageously cooled to from 220° C. to 280° C., preferably from 240°C. to 260° C., in a separate aftercooler which is connected downstreamof the reactor of the first stage.

[0185] The aftercooler is generally flanged on below the lower tubeplate and normally consists of tubes of ferritic steel. Stainless steelsheet metal spirals which may be partly or fully wound areadvantageously introduced into the interior of the tubes of theaftercooler, in order to improve the heat transfer.

[0186] Salt Melt:

[0187] The salt melt used may be a mixture of 53% by weight of potassiumnitrate, 40% by weight of sodium nitrite and 7% by weight of sodiumnitrate; both temperature zones and the aftercooler advantageously use asalt melt of the same composition; the amount of salt pumped bycirculation in the temperature zones may be approx. 10 000 m³/h perzone.

[0188] Flow Control:

[0189] The starting reaction gas mixture advantageously flows from topto bottom through the first stage reactor, while the salt melts havingdifferent temperatures of the individual zones are advantageouslyconveyed from bottom to top;

[0190] Catalyst tube and thermal tube charge (from top to bottom), forexample: Section 1: length 50 cm steatite rings of geometry 7 mm × 7 mm× 4 mm (external diameter × length × internal diameter) as a preliminarybed. Section 2: length 140 cm catalyst charge of a homogeneous mixtureof 30% by weight of steatite rings of geometry 5 mm × 3 mm × 2 mm(external diameter × length × internal diameter) and 70% by weight ofunsupported catalyst from section 3. Section 3: length 160 cm catalystcharge of annular (5 mm × 3 mm × 2 mm = external diameter × length ×internal diameter) unsupported catalyst according to example 1 of DE-A10046957 (stoichiometry: [Bi₂W₂O₉ × 2 WO₃]_(0.5)[Mo₁₂Co_(5.5)Fe_(2.94)Si_(1.59)K_(0.08)O_(x)]₁).

[0191] In the abovementioned charge, the unsupported catalyst fromexample 1 of DE-A 10046957 can also be replaced by:

[0192] a) a catalyst according to example 1c of EP-A 15565 or a catalystto be prepared in accordance with this example, except having the activecomposition Mo₁₂Ni_(6.5)Zn₂Fe₂Bi₁P_(0.0065)K_(0.06)O_(x).10 SiO₂;

[0193] b) example No. 3 of DE-A 19855913 as an unsupported hollowcylinder catalyst of geometry 5 mm×3 mm×2 mm or 5 mm×2 mm×2 mm;

[0194] c) unsupported multimetal oxide II catalyst according to example1 of DE-A 19746210;

[0195] d) one of the coated catalysts 1, 2 and 3 of DE-A 10063162,except applied in the same coating thickness to support rings ofgeometry 5 mm×3 mm×1.5 mm 7 mm×3 mm×1.5 mm.

[0196] According to the invention, the fixed catalyst bed isadvantageously otherwise selected in such a way (for example by dilutionwith, for example, inert material) that the temperature differencebetween the heating point maximum of the reaction gas mixture in theindividual temperature zones and the particular temperature of thetemperature zone generally does not exceed 80° C. This temperaturedifference is usually ≦70° C., frequently from 20 to 70° C., and thistemperature difference is preferably small. For safety reasons, thefixed catalyst bed is also selected in a manner known per se to thoseskilled in the art (for example by dilution with, for example, inertmaterial) in such a way that the peak-to-salt-temperature sensitivity asdefined in EP-A 1106598 is ≦9° C., or ≦7° C., or ≦5° C., or ≦3° C.

EXAMPLES

[0197] A reaction tube (V2A steel; external diameter 30 mm, wallthickness 2 mm, internal diameter 26 mm, length: 350 cm, and also athermal tube (external diameter 4 mm) centered in the middle of thereaction tube to receive a thermal element which can be used todetermine the temperature in the reaction tube over its entire length)is charged from top to bottom as follows: Section 1: length 50 cmsteatite rings of geometry 7 mm × 7 mm × 4 mm (external diameter ×length × internal diameter) as a preliminary bed. Section 2: length 140cm catalyst charge of a homogeneous mixture of 30% by weight of steatiterings of geometry 5 mm × 3 mm × 2 mm (external diameter × length ×internal diameter) and 70% by weight of unsupported catalyst fromsection 3. Section 3: length 160 cm catalyst charge of annular (5 mm × 3mm × 2 mm = external diameter × length × internal diameter) unsupportedcatalyst according to example 1 of DE-A 10046957 (stoichiometry:[Bi₂W₂O₉ × 2 WO₃]_(0.5) [Mo₁₂Co_(5.5)Fe_(2.94)Si_(1.59)K_(0.08)O_(x)]₁).

[0198] The first 175 cm from top to bottom are thermostatted by means ofa salt bath A pumped in countercurrent. The second 175 cm arethermostatted by means of a salt bath B pumped in countercurrent.

[0199] The above-described first reaction stage is continuously chargedwith a starting reaction gas mixture 1 of the following composition, andthe loading and the thermostatting of the first reaction tube arevaried:

[0200] from 6 to 6.5% by volume of propene,

[0201] from 3 to 3.5% by volume of H₂O,

[0202] from 0.3 to 0.5% by volume of CO,

[0203] from 0.8 to 1.2% by volume of CO₂,

[0204] from 0.01 to 0.04% by volume of acrolein,

[0205] from 10.4 to 10.7% by volume of O₂ and

[0206] the remainder ad 100% of molecular nitrogen.

[0207] The reaction gas mixture flows through the catalyst tube from topto bottom.

[0208] The pressure at the entrance to the reaction tube varies between2.3 and 3.1 bar as a function of the propene hourly space velocityselected.

[0209] At the exit of the reaction tube, a small sample is withdrawnfrom the product gas mixture for gas chromatography analysis. At the endof the temperature zone there is like-wise an analysis point.

[0210] The results as a function of catalyst velocities and salt bathtemperatures are shown by the following table 1 (the letter E inbrackets means example). TABLE 1 b) Results Propene hourly spacevelocity (l (STP)/ l · h) T_(A) T_(B) T^(maxA) T^(maxB) C^(P) _(A) C^(P)_(B) S^(AC) S^(AA) 130 (E) 319 319 384 351 66.7 95.1 92.6 5.1 130 (E)327 313 400 330 70.3 94.8 93.9 4.4 185 (E) 322 336 380 368 64.5 94.990.6 7.4

Comparative Examples

[0211] Everything is carried out as in the examples. However, the lengthof the charging sections of the fixed bed catalyst charges are changedas follows:

[0212] Section 2: 125 cm instead of 140 cm.

[0213] Section 3: 175 cm instead of 160 cm.

[0214] The results are shown by table 2. (C) means comparative example.TABLE 2 Propene hourly space velocity (l (STP)/ l · h) T_(A) T_(B)T^(maxA) T^(maxB) C^(P) _(A) C^(P) _(B) S^(AC) S^(AA) 130 (C) 319 315384 360 66.4 94.9 91.8 5.6 130 (C) 327 310 400 339 70.2 95.0 93.2 4.8185 (C) 322 331 380 377 64.7 94.8 89.9 7.8

[0215] Compared to the results in table 1, lower S^(AC) are achievedwith comparable C^(P) _(B) values.

1. A process for partially oxidizing propene to acrylic acid in the gasphase under heterogeneous catalysis by conducting a starting reactiongas mixture comprising propene, molecular oxygen and at least one inertgas, and containing the molecular oxygen and the propene in a molarO₂:C₃H₆ ratio of ≧1, in a reaction stage over a fixed catalyst bed whoseactive composition is at least one multimetal oxide comprising theelements Mo, Fe and Bi, in such a way that the fixed catalyst bed isarranged in two spatially successive temperature zones A, B, both thetemperature of temperature zone A and the temperature of temperaturezone B are a temperature in the range from 290 to 380° C., the fixedcatalyst bed consists of at least two spatially successive fixedcatalyst bed zones, and the volume-specific activity within one fixedcatalyst bed zone is substantially constant and increases sharply in theflow direction of the reaction gas mixture at the transition from onefixed catalyst bed zone to another fixed catalyst bed zone, thetemperature zone A extends up to a conversion of the propene of from 40to 80 mol %, on single pass of the starting reaction gas mixture throughthe entire fixed catalyst bed, the propene conversion is ≧90 mol % andthe selectivity of acrolein formation based on converted propene is ≧90mol %, the sequence in time in which the reaction gas mixture flowsthrough the temperature zones A, B corresponds to the alphabeticsequence of the temperature zones, the hourly space velocity of thepropene contained in the starting reaction gas mixture on the fixedcatalyst bed is ≧90 l (STP) of propene/l of fixed bed catalyst·h thedifference T^(maxA)−T^(maxB), formed from the highest temperatureT^(maxA) which the reaction gas mixture has within temperature zone A,and the highest temperature T^(maxB) which the reaction gas mixture haswithin temperature zone B is ≧0° C., wherein the transition fromtemperature zone A to temperature zone B in the fixed catalyst bed doesnot coincide with a transition from one fixed catalyst bed zone toanother fixed catalyst bed zone.
 2. The process as claimed in claim 1,wherein T^(maxA)−T^(maxB) is ≧3° C. and ≦70° C.
 3. The process asclaimed in claim 1, wherein T^(maxA)−T^(maxB) is ≧20° C. and ≦60° C. 4.The process as claimed in claim 1, wherein the propene hourly spacevelocity on the fixed catalyst bed is ≧90 l (STP)/l·h and <160 l(STP)/l·h.
 5. The process as claimed in claim 1, wherein the propenehourly space velocity on the fixed catalyst bed is ≧160 l (STP)/l·h and≦300 l (STP)/l·h.
 6. The process as claimed in claim 1, wherein thetemperature of reaction zone A is from 305 to 365° C.
 7. The process asclaimed in claim 1, wherein the temperature of reaction zone A is from310 to 340° C.
 8. The process as claimed in claim 1, wherein temperaturezone A extends to a propene conversion of from 50 to 70 mol %.
 9. Theprocess as claimed in claim 1, wherein temperature zone A extends to apropene conversion of from 60 to 70 mol %.
 10. The process as claimed inclaim 1, wherein the O₂:propene ratio in the starting reaction gasmixture is from 1 to
 2. 11. The process as claimed in claim 1, whereinthe chemical composition of the active composition used is unchangedover the entire fixed catalyst bed.
 12. The process as claimed in claim1, wherein the entire fixed catalyst bed comprises not more than 4 fixedcatalyst bed zones.
 13. The process as claimed in claim 1, wherein, whenthe active composition is uniform over the entire fixed catalyst bed,the volume-specific active composition in the flow direction of thereaction gas mixture increases by at least 5% by weight at thetransition from one fixed catalyst bed zone to another fixed catalystbed zone.
 14. The process as claimed in claim 1, wherein, when theactive composition is uniform over the entire fixed catalyst bed, thedifference in the volume-specific active composition between the fixedcatalyst bed zone having the lowest volume-specific activity and thefixed catalyst bed zone having the highest volume-specific activity isnot more than 40% by weight.
 15. The process as claimed in claim 1,wherein, when the active composition is uniform over the entire fixedcatalyst bed, the difference in the volume-specific active compositionbetween the fixed catalyst bed zone having the lowest volume-specificactivity and the fixed catalyst bed zone having the highestvolume-specific activity is not more than 40° C. by weight.
 16. Theprocess as claimed in claim 1, wherein the last fixed catalyst bed zonein the flow direction of the reaction gas mixture is undiluted andconsists only of shaped catalyst bodies.
 17. The process as claimed inclaim 1, wherein the fixed catalyst bed zone having the highestvolume-specific activity does not extend into temperature zone A. 18.The process as claimed in claim 1, wherein there is no transition fromone fixed catalyst bed zone to another fixed catalyst bed zone in thefixed catalyst bed within the range of $X \pm {L \cdot \frac{4}{100}}$

where L is the length of the fixed catalyst bed and X is the pointwithin the fixed catalyst bed of transition from temperature zone A totemperature zone B.